1. Field of the Invention
The present invention relates to a structure of a radial-gap type cylindrical motor, a method of mounting a salient pole piece thereon, and a method of improving the strength of the bobbin assembly.
2. Description of the Prior Art
In order to provide a high performance apparatus at low cost, such a system in which a plurality of motors specialized in respective functions are used in one apparatus, the so-called "sectional drive" system, has come into general use. There are demands for motors, especially for small-sized DC motors, that are light in weight, improved in motor efficiency with the magnetic circuit efficiency increased according to uses, having a lower cogging torque, generating small electromagnetic noise, and good controllability. As countermeasures for meeting these demands, such practices are being generally made to increase the number of slots of the core, increase the number of magnetic poles of the magnet to thereby decrease the length of each magnetic circuit and improve the magnetic efficiency, and to provide each salient pole with a coil of concentrated winding to thereby improve the efficiency of the winding work.
In the case of a flat, outer rotor type DC brushless motor widely used as the spindle motor for a magnetic memory device and the like, on account of a relatively small number of laminations of the core, construction of an armature structure having good efficiency/controllability was achieved as disclosed in Japanese Patent Application No. Hei 1-95928 (refer to the gazette of Japanese Patent Laid-open No. Hei 2-276449, hereinafter referred to as the prior art). More specifically, in view of its geometric configuration, it is very easy to set the sectional form characteristic coefficient R (=L.sup.2 /A), where A is the sectional area of the salient pole portion of the core and L is the length surrounding the salient pole portion, at such a value as satisfies the technological requirements described in the above patent application. Namely, it was easily attained to improve the motor performance by designing the sectional form of the salient pole portion to have a virtually square shape and allowing the flux density in the salient pole portion to match with the resistance value of the coil winding provided around the salient pole portion (to reduce the resistance value of each turn of the coil).
However, in the case of a motor for driving the carriage of a printer and the like, since the motor mounting space is small in spite of the relatively large mechanical output required of it, such measures are frequently taken to structure it in a cylindrical form and make the number of the laminations of the core as many as possible. In the design of the core of the DC brushless motors, a generally employed structure is such an armature that has laminations of a plurality of thin cores, including groove portions (hereinafter called "slots") for receiving coils and yoke portions for forming magnetic circuits (to be provided with concentrated coil windings and disposed in confronting relationship with the magnet across an air gap, hereinafter called "salient poles"), and is provided with insulation at necessary portions of the slot parts of the core and coil windings provided around the salient poles. Since this structure can change its characteristic only by laminating the needed number of cores of the same form in the axial direction, such an effect can be obtained, if there are, at hand, only the same form of metal die for pressing the cores, that the motor characteristic can be set at will by changing the number of the laminations. It further has such a feature that the iron loss is relatively low because the cores are electrically insulated.
However, in the laminated structure, especially that of a cylindrical type, the motor efficiency is liable to decrease greatly when the number of the salient poles is increased or the number of the poles of the magnet is increased. More specifically, in order to increase the mechanical output of the cylindrical type, it becomes necessary to increase the number of laminated cores. Although this method contributes to an increase in torque, the motor performance cannot be improved in spite of the increase in the number of laminated cores because of the following two reasons:
1) In general, it is preferred that the flux density in the salient pole portion is raised as high as just below the point of magnetic saturation. However, if the number of the laminated cores is increased to increase the torque, the sectional area of the yoke is also increased (the sectional area is proportional to the number of laminated cores) and, hence, the optimum flux density in the salient pole portion cannot be attained. Especially in the case of multipole/multi- slot structure, the width of the salient pole portion can be very small when the sectional area of the salient pole is considered. Then, however, the width becomes too narrow as against the core thickness and there arises such a difficulty that the core cannot be processed by stamping. Accordingly, the flux density in the salient pole portion cannot be increased to the optimum level. PA0 2) When the number of laminations is increased, the effective line element of the coil generating the torque becomes longer. However, the resistance value of the coil increases correspondingly and the motor efficiency decreases. Especially, such a coil winding provided around a narrow and long sectional area has a demerit that it only increases the resistance value and greatly deteriorates the motor efficiency.
Accordingly, in the laminated structure of the cylindrical type, the sectional form of the salient pole portion inevitably becomes worse (it becomes an oblong in which the difference between the long side and the short side is great). Hence, in the light of the prior art, it becomes unable to match the flux density in the salient pole portion with the resistance value of the coil provided around the salient pole portion. Namely, it becomes impossible to define the sectional area of the salient pole portion so as to provide the optimum flux density suitable for the motor. More specifically, there have been such problems to be solved that waste of the material of the stator is produced and, at the same time, the weight of the motor is increased and, further, the motor efficiency is deteriorated.
Furthermore, the electromagnetic noise produced at the time of commutation of the motor coil for use in either the magnetic memory device or the printer is regarded as harmful also by regulations in recent years. Therefore, it is desired that such noise is positively suppressed at a low cost.
The present invention is made in view of the above described situation and it is an object of the present invention to provide a motor structure, not using the structure to laminate a large number of cores of the same form in the axial direction, i.e., the so-called lamination structure, though it is of a radial gap, cylindrical motor, but being light, economical, high in performance, and generating a small amount of electromagnetic noise, and a method of mounting salient pole pieces thereon.